In the situation where a piston rides over discharge openings or passageways in a cylinder, for example, wherein said piston is provided with an elastic gasket which is fluid pressure medium, the problem is that said gasket may be extruded from the mounting groove and into the discharge passageway openings. This results in faster wearing of the gasket, leakage between the piston and the cylinder wall, and possible destruction of the gasket. To avoid this, it has been proposed that a housing casing be installed within the cylinder which has a number of small perforations in the area of the passageway openings. Such measures prevent the gaskets from being extruded into the discharge openings, but does not necessarily prevent damage to the gaskets.
The disadvantage of this proposal is that removing burrs and rounding-off the edges of the small perforations is a tedious and expensive process requiring much time, and there can be no assurance that some burrs will not remain to cause damage to the gasket.